This invention relates to a medical diagnostic ultrasound system and method for improving Doppler processing, such as color flow imaging. In particular, increasing the accuracy of Doppler estimates is provided by using signals at multiple frequency bands.
Doppler imaging comprises receiving signals in a single frequency band, such as at a fundamental frequency band or at a second harmonic frequency band of the fundamental frequency band. Typically, the frequency band is narrowly defined within the available broadband system bandwidth, increasing the amount of transmitted power and the signal to noise ratio (SNR).
One ensemble of received signals is processed to estimate motion at each point in space along an ultrasound line. This process is repeated over many ultrasound lines within an image frame.
Doppler imaging of tissue and blood is a qualitative diagnostic tool. The tool remains qualitative partly due to the inaccuracies of estimates. The demand for adequate frame rates, diagnostic resolution, and reasonable interrogated areas dictate that an ultrasound system typically only transmit 2 to 16 pulses to insonify any distinct point in space. This constraint may create estimate drop-outs in flow fields and demand extensive spatial and/or temporal averaging of estimates resulting in inaccurate differentiation between flow and tissue.
Some processes to increase sensitivity to Doppler velocities above conventional maximum velocity detection limits use signals at two different frequency bands. Conventional Doppler systems may not provide for accurate detection of velocities beyond a maximum value determined primarily by the center frequency within a single receive band. The sensitivity to velocities above this limit (the aliasing limit) is increased by using two different frequency bands. The difference between Doppler frequencies obtained from two separate narrow frequency bands is calculated. Since this difference Doppler frequency is less than the Doppler frequency from either of the two frequency bands independently, the maximum detect able Doppler frequency is extended.
For example, Powers describes in U.S. Pat. No. 4,534,357 a system that transmits broad, single band, pulses and then processes receive signals from two narrow receive bands for each broadband pulse transmitted. In other examples, Fehr in U.S. Pat. No. 5,046,500 and Burckhardt in U.S. Pat. No. 5,183,047 describe systems that transmit pulses composed of two separate narrow frequency bands and then process signals from a corresponding two narrow receive bands for each transmitted pulse. However, methods extend the maximum detectable velocities only.
In Ultrasound Imaging Enhancement Methods and Systems, by Ismayil Guracar et. al., U.S. application Ser. No. 08/838,920, now U.S. Pat. No. 5,961,460, filed Apr. 11, 1997, imaging enhancements using two different frequency bands are disclosed. In one embodiment, two different types of scan-converted images are combined into a single image by using a two-dimensional look-up-table (LUT). For example, an image of contrast agents corresponding to a harmonic frequency band is combined with an image corresponding to a fundamental frequency band. In one further embodiment in the above referenced application, "either or both of the fundamental and harmonic images are generated from signals detected by the Doppler detector 22 with no clutter filtering" (page 16).